The Labour leader said that the higher rate income tax threshold had stopped being a benchmark for "well off" thanks to a combination of rising childcare costs, soaring inflation and government cuts.

In a keynote speech, he warned of a "quiet crisis" in homes across Britain as middle earners realised they were no longer guaranteed prosperity.

The stark assessment came as Mr Miliband argued that the economy was failing to deliver for too many people after the financial crisis. He said that single-earner couples on £44,000 with children were set to lose thousands of pounds in benefit changes that failed the "fairness test".

Cuts to services, the VAT increase and childcare costs rising twice as fast as wages were leading to a "cost of living crisis for ordinary families".

"The Government is contributing to already high inflation and the squeeze on living standards with their decisions, and the crunch will be felt first and worst by low and middle income families, particularly those with children," Mr Miliband said as the Resolution Foundation think tank began an investigation into living standards.

He suggested that there was a "new inequality" between the richest at the top and the majority on lower and middle incomes who have been "struggling to keep up, working harder for less".

He said: "For many decades rising prosperity benefited the bulk of working people. While those at the top have continued to do well, middle earners are no longer guaranteed to share in our nation's success. The result is a quiet crisis that is unfolding day-by-day in kitchens and living rooms in every town, village and city up and down this country."

Moving the buckle on the 'squeezed middle'

Commentary: Joe Murphy, Political Editor

In today's speech, Ed Miliband is trying to reach out beyond his Red Ed stereotype. The bold message that people on £44,000 are not "rich" is designed to challenge the view that post-Blair Labour exists to help only the less well-off.

Mr Miliband makes a good argument that all except the very rich are being squeezed harshly. He believes a turning point will come when middle earners realise that, not only are they losing through taxes and cutbacks, but earnings have become stagnant.

Labour has also changed its rhetoric about benefits. Six months ago it was ramping up the unfairness of housing benefit cuts to Londoners but then realised most voters (including workers in the low-paid C2 and D social classes) approved of clamping down on claimants who get more from the State than they earn.

But it is too early to hail the rebirth of New Labour, the Blairite brand which the new leader declared to be dead last summer. Mr Miliband has yet to explain what he really stands for - as opposed to saying who he feels sympathy for - and how he would cut the deficit differently.

His much-maligned phrase "the squeezed middle" remains as fuzzy as ever. But while David Cameron is said to regard the middle classes as those on £25,000 to £40,000, Mr Miliband now appears to define them as earning from less than £20,000 to at least £44,000.